Institute of Electrical and Electronics Engineers (IEEE) 802.11s is a draft amendment to an international wireless LAN (WLAN) standard that describes a mesh network. WLAN devices in a mesh network are referred to as mesh points (MP). The MPs form mesh links with one another, over which mesh paths can be established using a routing protocol. The MPs have relay functions for wirelessly communicating directly with each other, instead of going through centralized control equipment such as a base station.
In a mesh network, data transmitted from one device may arrive at a destination via a sequence of MPs resulting in a multi-hop wireless network configuration. The mesh network of interconnected wireless links between the MPs enables automatic topology learning and dynamic path configuration/reconfiguration around inoperable paths by hopping from MP to MP until the destination is reached.
Communication between different MPs is controlled primarily through a Medium Access Control (MAC) protocol that uses IEEE 802.11 MAC/PHY layers to determine routes through the mesh network. In the MAC routing protocol, each MP learns about neighboring MPs using a neighbor discovery protocol based on received beacons or response management frames. Neighbors and neighbor's neighbors information is provided within the beacon management frames. Therefore, each MP can rapidly discover mesh points up to two hops away.
The MPs may be implemented as devices either plug-in AC powered devices such as a laptop, a microwave, and the like; or as battery operated DC powered devices such as a picture frame, a camera, and a cellular phone, for instance. MPs that have a connection to a power source may stay continuously awake, but battery operated MPs may optionally support a power save mode to conserve their battery.
IEEE 802.11 supports two power modes: active mode and power saving mode, assuming clock synchronization of the MPs is available. Synchronization may be achieved by each MP updating time stamp and offset information with information received in beacons and probe responses from other MPs. Once synchronized, time in the mesh network is divided into equal-length beacon intervals, each of which starts with an Ad Hoc Traffic Indication Message (ATIM) window. MPs are required to stay awake during the ATIM window so that control communication can be transferred between the MPs. For example, the ATIM window may be used by the MPs to indicate pending traffic, a change in PS state or re-instating stopped traffic flows. Any MP not receiving an ATIM frame within the ATIM window may enter the power save mode after the ATIM window expires. In certain circumstances, an MP receiving ATIM frames may remain active for a time after the ATIM window finishes in order to send or receive more data frames. MPs not in the power save mode may communicate with MPs in power save mode by buffering the data to be sent and then sending the data during another ATIM window.
Although the conventional power saving approach attempts to conserve power by requiring each MP to go to sleep after data transmission or reception, and wake up when there is data to be transmitted or received, the approach requires global scheduling because the MPs must synchronize and wake up at the same time to send or receive data. In addition to the drawback of requiring synchronization between the MPs, another drawback is that while the MPs are in power save mode, there is no connectivity between active MPs and the MPs in power save mode.